This invention relates to engine output control for a vehicle provided with an automatic transmission.
Tokkai Hei 1-206144 published by the Japanese Patent Office in 1989 discloses a hysteresis region provided in the upshift and downshift timing for an automatic transmission of a vehicle.
The shift timing of the automatic transmission is determined using the vehicle speed and the depression amount of the accelerator pedal as parameters. However when these parameters repeatedly fluctuate across the shift timing, the automatic transmission repeats a shift operation which results in a chattering phenomenon. The hysteresis region above has the function of preventing such chattering.
The operation of the hysteresis region allows a shift from a second gear to a third gear during acceleration for example to be performed at a greater vehicle speed than a shift from a third gear to a second gear during deceleration.
The drive force of a vehicle, that is to say, the output torque of the automatic transmission has an upper limit according to the gear ratio that the transmission applies.
For example, the upper limiting value of the output torque in the third gear is smaller than the upper limiting value of the output torque in the second gear. Thus even the accelerator pedal is depressed in a hysteresis region immediately before the automatic transmission shifts from the third gear to the second gear during deceleration, the output torque of the automatic transmission only increases to the upper limiting value in the third gear.
In other words, after the output torque of the automatic transmission reaches the upper limiting value, the output torque does not increase even with further depression of the accelerator pedal. In the following description, this state is termed saturation of output torque. Saturation of output torque causes the driver to feel that acceleration is not sufficient. Furthermore even when output torque is saturated, the driver will often attempt to increase the output torque by depressing the accelerator pedal further. As a result, there is a tendency for the throttle to become fully open when this is not required. When the throttle is fully open, fuel consumption by the engine increases and as a result exhaust emission performance deteriorates.
On the other hand, in the second gear, it is possible to obtain an output torque which is larger than the third gear. In order to solve the above problems, therefore, it has been proposed to advance the timing of a downshift from the third gear to the second gear during deceleration. However advancing the downshift timing tends to cause chattering in the shift operation and obviates the reason for setting the hysteresis region.
It is therefore an object of this invention to avoid saturation of the output torque of an automatic transmission when the acceleration pedal is depressed.
It is a further object of this invention to ensure the rate of increase of output torque of an automatic transmission when the accelerator pedal is deeply depressed.
In order to achieve the above objects, this invention provides a vehicle drive system, comprising an accelerator pedal, a sensor which detects a vehicle speed, a sensor which detects a depression amount of the accelerator pedal, an engine varying output torque in response to a signal, and a stepwise automatic transmission connected to the engine. The transmission performs an upshift operation at an upshift timing set according to the depression amount and the vehicle speed and a downshift operation at a downshift timing set according to the depression amount and vehicle speed. The vehicle speed corresponding to the upshift timing is set to be larger than the vehicle speed corresponding to the downshift timing for an identical depression amount.
The system further comprises a programmable controller programmed to compare the depression amount with a predetermined depression amount, set a first target output torque to a transmission target output torque of the automatic transmission when the depression amount is smaller than the predetermined depression amount, set a second target output torque to the transmission target output torque of the automatic transmission when the depression amount is larger than the predetermined depression amount, calculate an engine target output torque based on the transmission target output torque, and output the signal corresponding to the engine target output torque to the engine. Herein, the first target output torque increases at a predetermined rate of increase as the depression amount increases. The second target output torque increases at a rate of increase larger than the predetermined rate as the depression amount increases.
The programmable controller may alternatively be programmed to store a map of the transmission target output torque set to decrease as the vehicle speed increases, determine from a variation in the vehicle speed whether or not a vehicle is accelerating, calculate a modified vehicle speed, when the vehicle is accelerating, by subtracting a predetermined amount from the vehicle speed, calculate the transmission target output torque by applying the modified vehicle speed to the map, calculate an engine target output torque based on the transmission target output torque, and output the signal corresponding to the engine target output torque to the engine.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.